We let our son Jacob climb into his seat himself and he goes "wee" he stands and slides down.. He doesn't have an issue with being strapped in however.. Just with wanting to do everything himself..

I would just try and make a game out of it. it can get tiring however when he doesn't want to sit down straight away but we have found it makes life easier to take an extra 5 minutes getting him into the car..

Good luck.. toddlers sure are fun